How to apply a treatment Isocenter of a CT Volume in order to create a Digitally reconstructed radiograph (DRR)

I have a CT volume and I want to find the Isocenter. This is what I have done so far,

center_voxel_data_mm=[(size(voxel_data,1)/2)*voxel_header{1,1}.PixelSpacing(1),...
(size(voxel_data,2)/2)*voxel_header{1,1}.PixelSpacing(2),...
(size(voxel_data,3)/2)*voxel_header{1,1}.SliceThickness];

This is the true center of the CT volume I have also set an isocenter for this CT volume using a treatment planning machine (Pinnacle to be specific) The dicom coordinates are, x=-1.20mm,91.8mm,500.00mm. I apply the treatment isocenter as an offset to the true center of the CT volume, However when I create DRR using this new center it does not work. Does anyone have an idea what I might be doing wrong? I need help.